Connector for attaching a cable to a printed circuit board

ABSTRACT

An SMA PC mount connector for attaching a test cable for test equipment to a printed circuit board. The connector includes a conductor element having a reduced diameter so as to substantially eliminate impedance discontinuity resulting from the contact between the conductor element and the ground planes of the board at the mounting junction between the connector and the board to enable a more accurate measurement of the impedance of the true impedance of the board.

FIELD OF THE INVENTION

The present invention relates to connectors. In particular the presentinvention relates to an improved SMA PC mount connector for connecting acoaxial cable to a printed circuit board for connecting the circuitboard to test equipment, with the impedance discontinuity created by theconnector minimized to enable accurate detection of circuit generatedsignals on the printed circuit board by the test equipment.

BACKGROUND OF THE INVENTION

As part of development of frequency generation equipment integrated ontoa printed circuit board, such boards must be tested to determine thecircuit generated signals produced by the board. Typically such boardsare tested with an oscilloscope which measures the signal of the boardsand thus the frequency generation equipment mounted thereon over a rangeof frequencies. The boards generally are linked to the oscilloscope bycoaxial cables connected to the boards by SMA PC mount connectors.

FIG. 1A illustrates a conventional SMA connector 1 which includes a body2 having a mounting pad 3 of copper or similar conductive material atone end. A core 4 (FIG. 1B) formed from a dielectric material such asTEFLON® (polytetraflourene) or a similar material is positioned withinthe body and receives a conductor element 6 therethrough. Additionally,a series of locating spacer legs 7 (FIGS. 1A and 1B) are arranged aboutthe core, mounted to and extending forwardly from the mounting pad 3.The legs are used to help locate and mount the connectors to a printedcircuit board 8, as shown in FIG. 2, and to establish ground connectionsbetween the connector and the board. These conventional connectorsgenerally are mounted to the printed circuit boards 8 by the insertionof the legs 7 and conductor elements 6 of the connectors 1 throughopenings 9 formed in mounting pads 11 attached to the boards 8.Thereafter, the legs are soldered to the mounting pads 11 to fix theconnectors 1 to the boards for attaching a coaxial cable 12 thereto.

The primary problem with conventional SMA PC mount connectors is thatsuch connectors tend to create a substantial impedance discontinuity atthe connections of the connectors to the board. Such impedancediscontinuities are due to capacitances created between the legssurrounding the center conductor of the connector and the attachedstripline and the dielectric of the board. Also, these discontinuitiesresult from the surface area of the center conductors of such connectorscoupled to the ground planes of the board, the greater the surface areaof the conductor, the more capacitance that is generated, and thus thegreater the discontinuity. These impedance discontinuities in turn causedistorted signals to be observed on the oscilloscope that varysignificantly from the actual signal of the test board and which cannotbe statistically eliminated, especially during high-speed bandevaluations. For example, a reading of 25-30 ohms can be detected wherea desired target value of 50 ohms was expected, thus making it extremelydifficult to get a true reading of the signal distortion of the testboard. Thus, it generally has been difficult to determine a true measureof the actual signal as a result of the significant impedancediscontinuities created by conventional SMA connectors. Also, the centerconductors of such connectors generally have had to be relatively large,which further increases the impedance discontinuity due to capacitancecreated between the legs and the center conductor of conventionalconnectors.

Additionally, conventional Surface Mount Technology (SMT) connectorsgenerally require additional mounting pads to be applied on the boardsfor receiving mounting legs. The legs are then soldered directly to theboards. However, such soldering of the legs to the boards generallynecessitates the use of special soldering apparatus for soldering theconductors of the connectors to the boards, and provides only a limited,weak connection between the connectors and the boards. Such a weakconnection is a result of the legs providing limited flush mounting orengagement of the connector mounting pad against the board.Consequently, with only the soldered attachment of the legs to the boardholding the connector in place, the torque generated by repeatedscrewing of the cable couple onto the connector exerts a strain on theconductor of the connector, which can easily cause breaking of thesoldered connections of the legs and of the conductor of the connectorto the board. Additionally, the forces the cable exerts on the connectorcan weaken the board to connector engagement.

Accordingly, it can be seen that a need exists for an SMA connector forprinted circuit boards which reduces the capacitance effects at theboard connector mounting junction to reduce substantially the impedancediscontinuity thereat to enable a truer reading of the actual impedanceor signal distortion created by the printed circuit boards.

SUMMARY OF THE INVENTION

Briefly described, the present invention comprises an SMA PC verticalmount connector for connecting a coaxial test cable to a printed circuitboard for connecting the printed circuit board to an oscilloscope orsimilar testing equipment for testing, for example, frequency generatedequipment integrated on the printed circuit board. The connectorgenerally includes a substantially cylindrically shaped connector bodyhaving a proximal or first end and a distal or second end. The distal orsecond end of the connector body includes helical threads formedthereabout for attaching the coaxial cable to the connector. A centralcore formed from a dielectric material such as polytetraflourene,commonly known as TEFLON®, is received within the conductor body,extending partially along its length from its forward or proximal endtoward the distal end thereof. The proximal end further includes amounting pad formed as a substantially circular disc that radiallyoverlaps the proximal end of the connector.

A conductor element is mounted within the center of the dielectricmaterial of the core and typically includes a rod or pin that projectsoutwardly from the proximal end of the connector. A conductor sleeveextends from the rearward end of the conductor pin toward the distal endof the connector. The sleeve receives a conductor pin of the coaxialcable therein to establish an electrical connection. The conductorelement typically will be of a reduced diameter over conventional SMAconnectors and will be spaced from the mounting pad about the proximalend of the connector. Generally, a copper pad typically is mounted tothe printed circuit board at the point where the SMA PC mount connectoris inserted into the board. The conductor element further functions as alocating means for guiding the connector into engagement with the boardwith the mounting pad of the connector in substantially flush mountedattitude against the board mounting pad.

The use of a reduced diameter connector significantly reduces theimpedance discontinuity created between the conductor and the groundplanes of the printed circuit board to which the connector is mounted byreducing the surface area of the conductor element and thus reducing theelectrical field contact between the conductor element and the groundplanes of the board. The mounting pad at the proximal end of theconnector is soldered to the copper mounting pad of the printed circuitboard to establish a secure connection between the board and the SMA PCmount connector without requiring spacer or locating legs attached tothe mounting pad for mounting the connector to the board. Groundconnections are provided by connecting vias between the board groundplane and the mounting pad. The mounting pad and ground via connectioncombination allows the user to arrange the via geometry as needed inlieu of the restrictive through board legs of conventional SMAconnectors.

The construction of the present invention thus enables a truer measureof the impedance or signal distortion of the frequency generationequipment integrated on the printed circuit board, especially duringhigh speed band measurement operations of such digital systems, than canbe achieved with current conventional SMA connectors by the reduction ofimpedance discontinuities created by the contact of the electrical fieldof the conductor and the ground planes of the board. The elimination ofthe spacer legs further eliminates the incidence of impedancediscontinuities resulting from capacitances created between the legs andthe dielectric of the board and the center connector.

Various objects, features and advantages of the present invention willbecome apparent to those skilled in the art upon reading the followingdetailed description, when taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a perspective view of a conventional SMA PC mount connector.

FIG. 1B is an end view of the conventional SMA PC mount connector ofFIG. 1A.

FIG. 2 is a perspective view of a printed circuit board illustrating themounting of a conventional SMA PC mount connector thereto for connectinga cable to the printed circuit board.

FIG. 3A is a perspective view of the SMA PC mount connector of thepresent invention.

FIG. 3B is an end view of the SMA PC mount connector of the presentinvention.

FIG. 3C is a side view of the SMA PC mount connector of the presentinvention.

FIG. 4 is a perspective view of a printed circuit board illustrating themounting of the SMA PC mount connector of the present invention thereto.

FIG. 5 is a graphic illustration comparing the impedances measured on aprinted circuit board using a conventional SMA PC mount connector andSMA PC mount connector of the present invention.

DETAILED DESCRIPTION

Referring now in greater detail to the drawings in which like numeralsindicate like parts throughout the several views, FIG. 3A, 3B, 3C and 4illustrate the SMA PC mount connector 20 of the present invention foruse in connecting a coaxial test cable 21 (FIG. 4) to a printed circuitboard 22 having frequency generating equipment, indicated generally by23, mounted thereon to attach the printed circuit board to a test devicesuch as an oscilloscope (not shown). The printed circuit board generallyis a conventional type printed circuit board that includes an insulatingboard or card 24 formed from a dielectric material and on which copperconductors 26 are laminated or formed, such as by photochemical etching.The conductors include striplines 27 sandwiched between ground planesinternal to the board or card 24 of the printed circuit board. Thestriplines 27 generally are connected at their ends to mounting pads 28,each of which generally comprises a round shaped pad formed from copperor similar conductive material applied to the inner surface of theprinted circuit board where the stripline terminates. Each of themounting pads includes an aperture 29 formed approximately centrallytherethrough which is plated with copper or similar conductive material.

As shown in FIGS. 3A, 3B, and 3C the SMA PC mount connector 20 includesa connector body 31 generally formed from a metal such as brass platedwith gold or copper or similar conductive materials such as areconventionally used for such connectors. The connector body generally issubstantially cylindrically shaped and has a proximal end 32 and adistal end 33 and a substantially cylindrical side wall 34 that definesa longitudinally extending inner channel or passage 36 (FIG. 3A).Helical threads 37 are formed about the side wall 34 of the connectorbody extending from a point intermediate the proximal and distal ends tothe distal end 33 of the connector body 31. The threads enable theconnection of the test cable to the distal end of the connector. A core38 is received within and extends along the open ended channel 36substantially along the length of the connector body. The core generallyis formed from a dielectric material, typically a material such aspolytetraflourene, commonly known as TEFLON®, or similar insulatingmaterial.

A conductor element 41 is received approximately through the center ofthe core 38 and extends substantially along the length of the connectorbody 31. The conductor element includes a conductor pin 42 that projectsoutwardly from the proximal end of the connector body, and a sleeve 43that extends through the dielectric material of the core 38, terminatingadjacent the distal end 33 of the connector body 31. The sleeve 43receives the conductor of the test cable therein upon the attachment ofthe test cable to the SMA PC mount connector 20 to establish anelectrical connection between the conductor element 41 of the connector20 and the cable 21.

The conductor element 41 generally is formed from the same conductivematerial, i.e., brass plated with gold or copper, as the connector bodyand typically is of a diameter that is substantially less than thediameter of the conductor elements of conventional SMA PC mountconnectors. In general, the conductor element will be of a diameter lessthan 30 mils. or 0.030 inches or less, which is significantly less thanthe diameters of the conductor elements of conventional SMA PC mountconnectors. The conductor element 41 is received through an aperture 29(FIG. 4) of one of the mounting pads 28 mounted to the printed circuitboard 24, with the conductor pin 42 extending through the aperture andthrough a strip Line 27 to establish an electrical contact between thetest cable 21 and the printed circuit board 22. The conductor thus actsas a locating means to guide the connector into a flush mounted attitudein engagement with the board.

As illustrated in FIGS. 3A-4, a substantially circular shaped mountedpad 46 is formed at the proximal end 42 of the connector body 31 of eachSMA PC mount connector 20. The mounting pad generally is formed as partof the PC mount connector, forming the proximal end thereof and radiallyoverlaps or extends beyond the side wall 34 (FIGS. 3A and 4) of theconnector body 31. The mounting pad is adapted to engage and liesubstantially flat against a mounting pad 28 of the printed circuitboard 24 in a flush mounted attitude as indicated in FIG. 4 with theconductor pin 42 of the SMA PC mount connector 20 inserted through theconducting aperture 29 of such a board mounting pad 28. In such a flatlying condition, the mounting pads 46 of the SMA PC mount connectors 20can easily and quickly be soldered or otherwise connected, as indicatedby 47, to its board mounting pad 28.

Unlike conventional SMA PC mount connectors, as indicated in FIG. 1A andFIG. 1B, however, the connector 20 of the present invention does notinclude locating spacer legs mounted about and projecting forwardly fromthe mounting pad of such connector. Such spacer legs are not required asthe combination of the flat mounting pads of the SMA PC mount connector20 of the present invention and the mounting pads 28 affixed to the PCboard 24 to which the SMA PC mount connector of the present inventionconnection does not require additional openings to be drilled within thePC board for receipt of the locating spacer legs therein. Instead theflush mounting of the connector mounting pad against the board mountingpad provides a simple, stable and secure connection between theconnector and the board, and one which provides an excellent groundconnection between the connector and the board so as to substantiallyeliminate capacitance created by the fixed spacing of the legs to thecenter conductor of commercial SMA connectors. Such a flush mountingfurther resists torque and stresses on the solder connection between theboard due to the twisting attachment of the cable coupling to theconnector to resist breaking the connection between the conductor andthe board.

Typically, the SMA PC mount connector 20 is used in a vertical mountarrangement, indicated by 48 in FIG. 4, in which the connector isoriented vertically, perpendicular to the board. Additionally, the SMAPC mount connector of the present invention also can be used in an endlaunch mounting type arrangement indicated by 49 in FIG. 4. In such anend launch mounting arrangement, as opposed to the typical verticalmount arrangement for the connector, the connectors are mountedhorizontally with their conductor pins extending into a side edge of theprinted circuit board into engagement with a strip line 26 thatterminates at 45 at the side edge.

In use of the SMA PC mount connector 20 of the present invention, asshown in FIG. 4, the connector is mounted to the printed circuit boardby the insertion of its conductor element 41 through an aperture 29 of amounting of a board mounting pad 28 with the conductor pin engaging andextending through a strip line 27 of the printed circuit board and itsmounting pad 46 flush against the board mounting pad 28 in a verticalmount or end-launch type mounting and provide better mechanicalmounting. Once positioned in engagement with the board, the mounting pad46 further typically is soldered to the board mounting pad 28 to securethe connector to the board. In addition, the mounting pads 46 are beconnected to the ground planes of the board by VIAS 51 formed throughthe board mounting pads. Thereafter, the distal or female end of theconnector 22 is attached to the male connector at one end of the testcable, which typically is a coaxial cable, by the engagement of thethreads 37 formed about the connector body 31 adjacent the distal end 33of the conductor by the connector of the test cable. The opposite end ofthe cable is connected to test equipment, generally an oscilloscope, formonitoring the board's circuitry.

The reduced diameter of the conductor element 41 of the SMA PC mountconnector of the present invention advantageously enables thesubstantial elimination of the impedance discontinuity that generallyresults with commercial SMA PC mount connectors. The reduced diameter ofthe conductor element provides less surface area for the connectorcenter conductor element, which in turn reduces the capacitance createdbetween the conductor element and the board due to there being a reducedelectrical field contact between the conductor element and the groundplanes of the printed circuit board. As a result, the impedancediscontinuity generated by such capacitance is substantially eliminated.The substantial elimination of this impedance discontinuity thus enablesthe board to be designed with the ground planes closer to the centerconductor mounting point so as to enable better control of the impedanceof the strip lines themselves, without creating an increased impedancediscontinuity between ground planes and the conductor pins of theconnectors.

In addition, by doing away with the spacer legs required by mostconventional SMA PC mount connectors, there is a further reduction inthe impedance discontinuity created at the mount junction between theconnector and the printed circuit board due to the capacities createdbetween the spacer legs and the dielectric material of the PC boarditself and the center conductor. As a result, the impedancediscontinuities created between printed circuit boards used for thetesting of frequency generation equipment or the like and the PC mountconnectors used for connecting the test cable thereto for linking the PCboards to test equipment, such as an oscilloscope, are substantiallyeliminated. The SMA PC mount connector of the present invention thusenables a truer, more accurate reading of the actual signal distortionof the frequency generation equipment integrated on the printed circuitboards. The printed circuit boards therefore can be designed and builtto a specific desired impedance, which enables greater standardizationin the construction of such printed circuit boards to match desiredimpedances.

As illustrated graphically in the FIGS. 5, in tests for measuring theimpedance of a printed circuit board having the SMA PC mount connectorof the present invention versus a commercial SMA PC mount connectormounted thereto to connect the test cables to the board, the impedanceof the printed circuit board using the SMA mount connector of thepresent invention detected by the digital sampling oscilloscope wasfound to be much closer to the desired target impedance for the boardthan the impedance detected for a board connected thereto using acommercially available SMA PC mount connector. In FIG. 5, the ordinateillustrates the deviation from the norm of the impedance, while theabscissa illustrates the length of the discontinuity. The target valueof the desired impedance for the printed circuit boards is approximately50 ohms, which is generally indicated by the horizontal line labeled ρ1.The curving line indicates the measured impedance of the board, whichrises sharply to indicate the end of the connector on the backside ofthe board.

FIG. 5 illustrates the difference in the measurement of impedancesbetween a commercially available SMA connectors, two modified SMA PCmount connectors of the present invention and a reference value of theimpedance of the cable itself. In this figure, the upper horizontalline, indicated by ρ1, indicates the desired target impedance for theboard. The second, lower horizontal line, indicated by the ρ2, indicatesthe depth of the impedance discontinuity from the desired targetimpedance of the board measured on a board using a conventional SMA PCmount connector. As indicated in FIG. 5, the impedance as measured onthe boards connected with the modified SMA PC mount connectors of thepresent invention much more closely track the target impedance value,whereas there is a wide discrepancy, evidenced by the space between thetwo lines ρ1 and ρ2, between the target impedance value and the measuredimpedance value measured on a PC board using a convention SMA PC mountconnector.

Thus, it can be seen that the present invention enables a much moreaccurate determination of the signals of a printed circuit board withonly a minor deviation resulting from the mount junction of the SMA PCmount connector of the present invention to the board, which smalldiscrepancy can be statistically eliminated so as to enable the printedcircuit boards to be designed and built to a desired impedance on a muchmore standardized basis.

It will be understood by those skilled in the art that although theforegoing description describes the invention in detail with referenceto a preferred embodiment thereof, various modifications, additions anddeletions can be made thereto without departing from the spirit andscope of the invention as set forth in the following claims.

I claim:
 1. A connector for attaching a cable to a printed circuit boardto connect the printed circuit board to test equipment for measuring theimpedance of the printed circuit board, with the impedance discontinuityresulting from a mount junction between the connector and the boardbeing substantially eliminated to enable a reading of the impedance ofthe impedance frequency generation equipment of the board with increasedaccuracy, comprising:a connector body having an outer sidewall, aproximal end, and a distal end adapted to attach to the cable; a core ofa dielectric material received within said connector body between saidproximal and distal end thereof, a conductor element received andextending through said core spaced from said outer side wall of saidconnector body and having a diameter of less than approximately 30 mils;and means for reducing impedance discontinuities between said connectorand the board, said means comprising:a mounting pad attached to saidconnector body at said proximal end thereof and adapted to engage andlie substantially flush against a mounting pad for the printed circuitboard; and means for securing the connector to the printed circuit boarddisposed at said engagement of said connector body mounting pad and saidprinted circuit board mounting pad and being substantially confinedthereat.
 2. The connector of claim 1 wherein said core of dielectricmaterial comprises a substantially cylindrical body formed frompolytetraflourene.
 3. The connector of claim 1 and wherein saidconductor element includes a pin conductor that projects from saidproximal end of said connector body and a sleeve portion at said distalend of said connector body.
 4. The connector of claim 1 and wherein saidconductor and said connector body are formed from brass.
 5. An SMA mountconnector for attaching a test cable to a printed circuit board havingfrequency generation equipment integrated thereon to connect the printedcircuit board to a measurement device for measuring the signaldistortion of the frequency generation equipment of the board, with theimpedance discontinuity resulting from the creation of localizedcapacitances resulting from the connection of the connector to the boardbeing substantially eliminated to enable a substantially accuratemeasurement of the signal distortion, and thus the impedance of theboard by the test equipment to enable more standardization of theconstruction of the board having a desired impedance, comprising:aconnector body having proximal and distal ends; a core of dielectricmaterial received within said connector body; a connector mounting padformed at said proximal end of said connector body, circumscribed aboutsaid core and adapted to engage and lie substantially flat against theboard for attachment of the connector to the board; a board mounting padapplied to the board and adapted to receive said connector mounting padin a substantially flush mounted attitude thereagainst for attachingsaid connector mounting pad to mount the connector to the board, andhaving an aperture therethrough; and means for locating said aperture ofboard mounting pad for guiding the connector into its flush mountedattitude with the board, comprising a conductor element received andextending though said core of said conductor body and having a diameterso as to minimize capacitances existing between the board and saidconductor to enable a more accurate measurement of the impedance of theboard for constructing the board with a desired impedance.
 6. Theconnector of claim 5 and wherein said core of dielectric materialcomprises a substantially cylindrical body formed frompolytetrafluorine.
 7. The connector of claim 5 and wherein saidconductor element includes a pin conductor that projects from saidproximal end of said connector body and a sleeve portion at said distalend of said connector body.